ABSTRACT Circulating tumor cells drive metastasis when they travel from primary tumors to distant organs via the circulation. Multicellular clusters of circulating tumor cells though less frequently observed in blood, are much more likely to establish metastases than individual circulating tumor cells and the presence of tumor clusters in blood has been associated with dramatically worse prognoses in patients. Although there are many suspected explanations for their greater metastatic potentials, much is still unknown about the behavior of clusters, especially in the narrow vessels of the body. Recent evidence has demonstrated that cluster transiting through narrow constrictions experience dynamic changes to structure and organization. Forces in the microcirculation cause clusters to reversibly re-organize into single-file chains to enable transit through narrow capillary-sized vessels and nuclear envelopes are ruptured and rapidly repaired during migration events through narrow constrictions. Two biophysical parameters within clusters, cellular adhesion strengths and nuclear mechanics, are vital for these behaviors. Because of the important role that these parameters play in many aspects of metastatic progression, we hypothesize that these parameters modulate the biophysical responses of clusters to physical forces in the microcirculation, and that these interactions play a significant role in the competitive edge that clusters have edge over individual cancer cells for seeding metastases. To this end, we propose three specific aims. In aim 1, we will develop next generation models of the human microcirculation with rounded networks of endothelial cell coated microfluidic devices and geometry matched computational simulations. In aim 2, we will explore how intercellular adhesions affect the biophysical responses and metastasis-forming abilities of homogeneous versus heterogeneous clusters in the microcirculation through the use of our developed models. Finally, in aim 3 we will study the physical basis for nuclear envelope rupture, DNA-damage, genetic instability and other DNA-level affects that are involved in metastatic progression. Understanding the interplay between the biophysics and biology of clusters within the microcirculation will elucidate mechanisms that can be used to combat the progression of cluster-initiated metastases.